Esters are produced by the reaction of an alcohol with a carboxylic acid or a carboxylic acid anhydride. In many instances, one or more of the starting materials may be mixtures. The carboxylic acid may be a mono or a polycarboxylic acid or the anhydride thereof. Plasticiser esters are generally produced from polycarboxylic acids or the anhydrides thereof and in particular from phthalic anhydride, cyclohexane dicarboxylic acid or its anhydride, terephthalic acid, adipic acid or trimellitic acid or anhydride. Esters of benzoic acid or the anhydride thereof, such as isononyl benzoate or isodecyl benzoate, are examples of plasticiser mono-esters. Esters may be produced from any alcohol, but plasticiser esters are generally produced from C4 to C13 alcohols and in particular C6 to C13 alcohols more typically C8 to C10 alcohols.
The production of esters and in particular plasticiser esters are described in patents too numerous to list. Examples include U.S. Pat. Nos. 5,324,853; 5,880,310; 6,310,235, and 6,355,817, and PCT publications WO 2006/077131 and WO 2006/125670.
US 2004/0015007 discloses isomeric nonyl benzoates and mixtures thereof with alkyl phthalate, alkyl adipate or alkyl cyclohexane dicarboxylate. The mixtures disclosed contain at least 1% by weight of isomeric isononyl benzoates and US 2004/0015007 is silent about the possible presence of benzoates derived from C10 alcohols.
Isodecyl benzoates are disclosed in U.S. Pat. No. 5,236,987 and US 2007/0010599. These publications are silent about possible mixtures thereof with other esters, such as alkyl phthalates, adipates or cyclohexane dicarboxylates.
A specific isodecyl benzoate, including from 50 to 99% of 2-propylheptyl benzoate and from 1 to 50% of at least one selected from a group of other specific isodecyl benzoate isomers is disclosed in US 2004/0138358. Further disclosed are compositions including from 5 to 90% by weight of this specific isodecyl benzoate and from 10 to 95% by weight of another ester selected from a di-C4-C13 alkyl phthalate, such as di-isononyl phthalate, a di-C4-C13 alkyl adipate, such as di-isononyl adipate and di-C4-C13 alkyl cyclohexane dicarboxylate, such as di-isononyl cyclohexane dicarboxylate.
Plasticiser esters may be produced in batch mode. Due to their high production volumes, they are preferably produced in continuous mode or in semi-continuous mode. The esterification process is particularly suited for production in semi-continuous mode. In semi-continuous esterification operations, the esterification reactors are typically operated in batch mode, the crude ester products from the different reactor batches then being collected and processed in continuous mode through a finishing section for product clean-up and purification. Suitable esterification processes are also disclosed in co-pending U.S. patent applications 60/906797 or 60/906732. The finishing section typically comprises catalyst removal, neutralisation of leftover traces of acid, separation of catalyst residues and/or salts from the neutralisation, separation and recovery of excess alcohol. A detailed description of an esterification finishing section and how it may be operated is disclosed in WO2005/021482 or WO2006/125670.
Operating the finishing section in continuous mode creates a problem when more than one ester product is produced in the same production facility, and when the esterification process has to switch from one product to another on a regular basis. A conventional method is to shut down and empty the finishing section of the process, and clean the equipment by e.g. a water wash before the next reaction product is fed to the finishing section. This introduces a significant amount of down time, and possibly generates substantial volumes of waste water that needs to be disposed of. An alternative is a dry changeover, thus eliminating the use of water, but which because of grade cross-contamination typically produces a certain volume of mixed grade product material, which is generally not compliant with any of the individual product grade specifications and therefore may need to be downgraded to a lower sensitivity end-use or be reworked.
The amount of such cross-contaminated or mixed grade product material may be significant, because the liquid hold-up in the equipment of the continuous finishing section may be important. Contributing to this liquid hold-up may be the liquid contained in any of the following equipment, when present:                the feed vessel to the finishing section;        the neutralisation vessel, wherein acids remaining from the esterification reaction are neutralised with a base, and wherein the catalyst (e.g., titanate catalyst), is removed such as by hydrolysing;        the water separator vessel conventionally placed after the primary filtration step, if the wet titanium process is used;        the water flash vessel, if the dry titanium process is used;        the filter aid mixing vessel, in the case where a primary filtration step using filter aid is included in the process;        the precoat vessel to the primary filtration, where precoat material, typically an inert material of a suitable granularity, may be mixed into product liquid before circulating this over a fresh primary filter to deposit the layer of precoat filter material on the filter before commisioning;        the primary filters themselves, when present, including the liquid contained in the filter cake;        the stripper, where excess alcohol may be stripped from the crude ester, typically after the primary filtration step, by using steam or nitrogen;        the product drying tower, when used, where steam stripped ester may be further dried in countercurrent with nitrogen;        the filter aid mixing vessel, precoat vessel and the filters comprised in a secondary filtration step, when used.        
There remains therefore a need for a method for switching production from one product grade to a second product grade in an esterification process having a continuous finishing section, maximising equipment productivity while minimising product cross-contamination. The present inventors have found that such productivity benefits may be achieved, while keeping product contamination to an acceptable level, by applying a so-called “flying grade switch procedure”. Moreover, in embodiments, co-products from an esterification process employing such flying grade switch procedure on particular product grade sequences bring performance benefits.